The invention relates to a device and a method for preventing a tool from breaking during fine blanking and/or forming in a press. A device for protecting a tool set of a hydraulic fine blanking press is known from DE 24 19 390 C2, in which a ram of a press can be lifted using quick-action cylinder-piston units and a working stroke pressure unit. A sensing gap exists in the press between the ram driving element and a driven ram part. The sensing gap is maintained when a pressure means approaches. The gap closes when a higher pressure load is applied to the parts forming the gap. The state of the sensing gap is monitored with a feeler and with a limit switch until the ram has caused the tools to approach each other to an extent where a proper start of the working stroke is reached. The sensing gap is formed between the ram and the end faces of the piston rods of the quick-action pistons. A pressure means is applied to the sensing gap in sensing cylinders in the ram. The sensing pistons in each case are seated against a partial surface of these end faces.
In this known protective device, the sensing gap in front of the quick-action cylinder-piston unit is associated with the ram and is connected upstream of the ram. This protective device is activated when the sensing gap closes due to an increased pressure load.
It has the disadvantage that stamping scrap, parts or foreign objects in the tool cannot be detected, and consequently damage to the tool cannot be prevented.
DE 69 17 177 U discloses another solution for protecting tools on presses for processing a blank. Two switches, between a first tool moved by the main piston and a second tool that is rigid with the press frame, are actuated in a predetermined sequence so as to interrupt the working stroke when the actuating sequence is reversed. An element is provided, which is used to attach the first tool and can be axially displaced in relation to the main piston. The element is held a small distance in front of the main piston by an elastically yielding means during the working stroke. A first limit switch, which can be set to the target height of the blank to be processed, is actuated by a stop when the piston has traveled a portion of the working stroke thereof that is predetermined by the setting of the first limit switch. A second limit switch is actuated by a further stop when the element has been pushed back by the small distance onto the piston due to impingement of the blank or a foreign object on the second tool.
The element is designed as a lower tool clamping table, which rests on a cylinder. The cylinder together with the main piston, encloses an annular pressure chamber connected to a pressure means line. A blocking means is located in the course of the pressure means line which allows sufficient compressed air to be admitted into the annular chamber, so that the table is practically located in a floating position in front of the piston during the working stroke.
This known solution has the drawback that, in addition to the hydraulic system, a second separate system is required, which operates with compressed air. It thus entails considerable switching and design complexity. In addition, after a foreign object has been detected, there is no option to dissipate the kinetic energy of the drive system.
The prior art from which the present invention proceeds is described in EP 2 258 496 A1, which relates to a device and to a method for preventing a tool, composed of an upper part and a lower part, from breaking during fine blanking and/or forming in a press. The known device comprises a main cylinder, which is disposed in the tine blanking head and in which a V-ring piston located on the stroke axis and multiple V-ring pins disposed coaxially to the stroke axis are accommodated. The V-ring pins are axially guided in a main plate closing the main cylinder and are supported on a V-ring pressure plate. The main cylinder is held in the head piece of the press and is closed in a pressure-tight manner by a V-ring cover. The fine blanking head is connected to a hydraulic system for generating pressures acting on the tool by way of valves that can be actuated by a computer.
A sensing piston is provided between the V-ring piston and the V-ring pin. The piston forms an axially displaceable structure together with the V-ring piston on the one hand, and together with the V-ring pin and the V-ring pressure plate on the other hand. The V-ring piston is associated with a pressure chamber that can be acted on by a preload pressure of the hydraulic system and with a pressure chamber that can be acted on by a sensing piston pressure between the V-ring piston and the sensing piston. The sensing piston is associated with a further pressure chamber that can be acted on by a pressure differential. The pressure chambers in each ease are connected via a connection in the V-ring cover to the hydraulic system for acting on the V-ring piston and the sensing piston counter to the operative direction of the preload pressure.
Moreover, a sensing table is provided, which is held in a suspended and weight-compensated manner at a defined sensing distance by the main plate counter to a clamping means engaging on the main plate. A sensor is associated with the sensing distance, which forwards a signal to the computer to immediately stop the press so as to protect the upper tool part when the sensing distance changes.
This known prior art has the drawback that the sensing table represents an additional component, which results in an increase of the mass to be moved, and thus in reduced sensitivity of the sensing device. By arranging the sensing table below the main plate, moreover the distance of the measuring point for ascertaining stamping scrap, parts that were left behind or faulty adjustments in the tool installation height from top dead center is increased. The result is a higher ram speed, which reduces the sensitivity of the sensing process.